How the Patellofemoral Joint Works and Why It Causes Pain

The patellofemoral joint is where the kneecap (patella) rides against the front of the thighbone (femur), and it absorbs remarkable forces during everyday activities. Walking on flat ground generates a compressive load on this joint equal to roughly half your body weight, but climbing stairs can push that load to several times body weight. Because the kneecap sits in a shallow groove and depends on muscles, ligaments, and bony geometry to stay centered, it is one of the most common sources of knee pain in both athletes and sedentary adults. Understanding how this joint works, what goes wrong, and what actually helps is more nuanced than most people realize.

How the Kneecap Actually Works

The patella is the largest sesamoid bone in the body, meaning it is embedded within a tendon rather than connected directly to other bones at a traditional joint. It sits inside the quadriceps tendon and connects below to the patellar tendon, which attaches to the shinbone. When you bend and straighten your knee, the kneecap slides up and down within a groove on the femur called the trochlear groove. Its primary job is to act as a lever arm for the quadriceps muscles, increasing their mechanical advantage by holding the tendon farther from the center of rotation of the knee. Without a kneecap, your quadriceps would need to generate substantially more force to extend the knee.

The contact between the kneecap and the groove changes depending on how bent your knee is. At low angles of bending, only a small area of the kneecap touches the femur. As the knee bends further, the contact area grows. During stair descent at about 30 degrees of flexion, one study measured a contact area of roughly 250 square millimeters, while during stair ascent at about 60 degrees the area nearly doubled to around 434 square millimeters.¹ This is relevant because when the contact area is small, the same amount of force gets concentrated into a smaller patch of cartilage, which can contribute to damage over time.

Why So Many People Get Patellofemoral Pain

Patellofemoral pain syndrome is one of the most common diagnoses in sports medicine clinics, and it is not limited to runners or athletes. The hallmark symptom is a dull, aching pain around or behind the kneecap that worsens with activities like squatting, climbing stairs, sitting for long periods with bent knees, or running. It is sometimes called “runner’s knee,” though that label oversimplifies both who gets it and what causes it.

The pain does not come from one single source. In some people, cartilage on the back of the kneecap softens and breaks down, a condition called chondromalacia patellae. This can be reversible if caught early, or it can progress toward patellofemoral osteoarthritis.² In other people, the bone underneath the cartilage appears to play a role. Research suggests that pain fibers in the subchondral bone respond to increased water content and intraosseous pressure during loading, meaning the bone itself can be a pain source even when cartilage looks relatively intact.³

The Role of Patellar Tracking

A major contributor to patellofemoral problems is how well the kneecap tracks in its groove. Ideally, the patella glides straight up and down as the knee bends and straightens. In many people with pain, the kneecap tilts or shifts to one side, typically laterally (toward the outside of the knee). This “maltracking” changes where forces are concentrated on the cartilage.

Muscle imbalances are part of the story, but the picture is more specific than the old advice to “just strengthen your quads.” In people with patellofemoral pain who also have abnormal patellar tracking, delayed activation of the inner thigh muscle (vastus medialis) relative to the outer thigh muscle (vastus lateralis) is strongly correlated with the degree of maltracking.⁴ The ratio of outer-to-inner quad activation also correlates with patellar tilt, but only in those patients classified as maltrackers.⁵ In pain-free people and even in some pain patients without measurable maltracking, these muscle-timing differences do not show up. That is a useful reminder that not all patellofemoral pain has the same underlying driver.

How Hip Weakness and Foot Mechanics Feed Into Knee Pain

The patellofemoral joint does not operate in isolation. It sits in the middle of a chain that runs from the hip above to the foot below, and problems at either end can change the forces on the kneecap. This is one of the most practically important things to understand about patellofemoral pain, because it means the treatment sometimes needs to target the hip or foot rather than the knee itself.

Weakness in the hip abductors and external rotators allows the thigh to rotate inward and collapse toward the midline during single-leg activities like running, stair climbing, or squatting. This increases the so-called Q-angle, which is the angle between the pull of the quadriceps and the line of the patellar tendon. A systematic review with meta-analysis found strong evidence that people with patellofemoral pain have reduced hip external rotation strength and reduced hip abduction strength compared to pain-free controls.⁶ One study of women with the condition found they demonstrated about 14% less hip abductor strength and 17% less hip extensor strength, along with greater peak hip internal rotation during movement.⁷

A systematic review found that a 10-degree increase in the Q-angle can raise lateral contact pressure on the patellofemoral joint by about 45%.⁸ Lab studies confirm that increasing the Q-angle shifts the patella laterally and can create conditions favorable to lateral dislocation or increased lateral contact pressures.⁹

At the other end of the chain, excessive foot pronation (rolling inward at the ankle) rotates the shinbone internally, which also pushes the kneecap’s tracking environment in a less favorable direction. Foot orthoses may help some people with patellofemoral pain by reducing internal rotation of the lower extremity and decreasing the Q-angle.¹⁰ A systematic review found limited evidence that prefabricated foot orthoses may provide short-term improvements, though the mechanism linking foot mechanics to patellofemoral pain development remains unclear.¹¹ Custom-fitted orthoses may improve symptoms specifically in people who demonstrate excessive pronation.¹²

Exercise Rehabilitation and How to Choose Between Hip and Quad Work

The evidence on exercise for patellofemoral pain is both encouraging and a bit anticlimactic. Strengthening exercises work, but the specific type matters less than most people expect. A well-designed equivalence trial comparing 12 weeks of quadriceps-focused exercises to hip-focused exercises found that both approaches produced similar improvements in pain and function, with the difference between groups falling well within an equivalence margin.¹³ Another randomized trial in women found that hip strengthening was not more effective than quadriceps or stretching exercises for pain relief and function, though only the hip and quadriceps groups succeeded at reducing dynamic knee valgus (the inward collapse of the knee during single-leg tasks).¹⁴

Where it gets more interesting is a secondary analysis that looked at which patients responded better to which type of exercise. People who catastrophized about their pain or were overweight tended to get more benefit from hip-focused exercises, while those with more severe knee pain tended to do better with quadriceps-focused exercises.¹⁵ This suggests that matching the exercise approach to the individual may matter more than picking a universally “best” program.

Taping and Bracing for Short-Term Relief

Patellar taping is commonly used as an adjunct to exercise therapy, and the evidence supports it for short-term pain reduction, though the mechanism is debated. A systematic review and meta-analysis found moderate evidence that tailored patellar taping, customized to the individual to control lateral tilt, glide, and spin, provides an immediate large pain reduction, while untailored taping provides a smaller effect. Tailored taping also promoted earlier onset of the inner quad muscle contraction relative to the outer quad. When combined with exercise, tailored taping showed superior pain reduction compared to exercise alone at four weeks, but the added benefit washed out by three to twelve months.¹⁶

There are different taping philosophies. McConnell taping aims to physically shift patellar alignment, while Kinesio taping works on muscle facilitation. A meta-analysis found that Kinesio taping relieves pain but does not change patellar alignment, whereas McConnell taping does both. Both techniques improved muscle activity and function.¹⁷ One study also found that a tibial rotation-limiting tape technique reduced pain during lunges and single-leg squats to a similar degree as the traditional patellofemoral joint taping, though neither technique altered lower limb movement patterns in that study.¹⁸ The upshot: taping is useful as a bridge that makes exercise more tolerable, but it is not a standalone solution.

Patellar Instability and Dislocation

Beyond pain from overuse or malalignment, some people experience the kneecap actually sliding out of its groove, which is a patellar dislocation. This is a distinct problem from patellofemoral pain syndrome, though the two can coexist. The three primary anatomical risk factors are trochlear dysplasia (a shallow or misshapen groove), patella alta (a kneecap that sits higher than normal), and an increased distance between the tibial tuberosity and the trochlear groove (TT-TG distance).¹⁹ ²⁰

The primary soft-tissue restraint keeping the kneecap from sliding laterally is the medial patellofemoral ligament (MPFL), which provides roughly 50 to 60% of the total restraining force against lateral displacement during the first 0 to 30 degrees of knee bending.²¹ When this ligament tears during a dislocation, the risk of recurrence climbs, especially in younger patients and those with the anatomical risk factors mentioned above.

Imaging plays a key role in surgical planning. The TT-TG distance can be measured on both CT and MRI with good reliability, but the values from these two methods are not interchangeable. MRI may underestimate TT-TG distance compared to CT, which is important to know if surgical correction is being considered.²² Normal TT-TG values measured on MRI average about 10 millimeters.²³

When Surgery Comes Into the Picture

Most patellofemoral pain responds to conservative treatment, and surgery is typically reserved for recurrent patellar instability or patellofemoral osteoarthritis that has not improved with rehabilitation. For instability, the two most common procedures are MPFL reconstruction (rebuilding the torn ligament with a graft) and tibial tubercle osteotomy (moving the bony attachment point of the patellar tendon to improve alignment). These are sometimes combined when a patient has both ligament insufficiency and bony malalignment.

A retrospective study of 23 patients who underwent combined MPFL reconstruction and tibial tubercle osteotomy found a low recurrence rate of about 4%.²⁴ A meta-analysis comparing MPFL reconstruction alone versus the combined procedure found that functional outcome scores (Kujala and Lysholm) were similar between groups, but isolated MPFL reconstruction had a higher surgical failure rate (about 5% versus 3%) and the combined procedure had a higher rate of secondary surgeries.²⁵ A separate meta-analysis found that isolated MPFL reconstruction led to similar pain and redislocation rates as the combined procedure even in patients with TT-TG distances over 15 millimeters, though return-to-sport rates were lower.²⁶

For isolated patellofemoral osteoarthritis, partial joint replacement (patellofemoral arthroplasty) is an option that preserves the rest of the knee. A systematic review found that in the first two years after surgery, patellofemoral arthroplasty provided higher activity levels and better functional recovery than total knee arthroplasty, with no significant difference in complications or satisfaction.²⁷ Longer-term follow-up at a minimum of eight years told a more complicated story: function and satisfaction scores were similar between the two procedures, but the partial replacement had lower survival (about 92% at ten years versus 100% for total knee arthroplasty), though this difference was not statistically significant.²⁸ Patellofemoral arthroplasty is a reasonable option for the right candidate, but the trade-off between early recovery advantages and a slightly higher long-term revision risk is worth discussing with a surgeon.

Squatting, Loading, and Exercise Modification

Squats are one of the activities people worry about most when they have patellofemoral pain, and for good reason. Patellofemoral joint force and stress both increase as the knee bends deeper, with peak values at around 90 degrees of flexion.²⁹ Going below parallel with heavier loads pushes these forces higher still.³⁰ That does not mean squatting is bad for the joint. Cartilage adapts to loading, and many rehabilitation programs include progressive squatting. But if you are managing patellofemoral pain, limiting squat depth to roughly parallel or slightly above, and gradually increasing load, is a sensible approach during the symptomatic phase.

It is also worth noting that peak patellofemoral stress did not vary significantly between different squat stances (narrow, shoulder-width, and wide) in one study of collegiate women athletes.³¹ So changing your stance width probably does less for patellofemoral stress than controlling depth and load does.

Sex Differences and Hormonal Influences

Patellofemoral pain is more common in women, and several factors contribute. Wider pelvis geometry creates a naturally larger Q-angle, hip weakness patterns are more prevalent, and hormonal fluctuations add another layer. Knee laxity changes meaningfully across the menstrual cycle: during ovulation, knee laxity increases and stiffness drops by roughly 17% compared to the luteal phase.³² Research has shown that estradiol, progesterone, and testosterone all contribute to this variation, with laxity changes following hormone shifts by three to four and a half days on average. In one study, these hormones and their interactions explained an average of 63% of the variance in knee laxity.³³

Sex differences in knee laxity are also menstrual-cycle dependent. Women have greater laxity than men at multiple time points throughout the cycle, with the differences coinciding with elevations in estradiol.³⁴ Increased laxity does not automatically mean more pain, but it likely changes patellar tracking behavior and may partly explain why women report more patellofemoral symptoms during certain phases of their cycle. This is an active research area and not yet well enough understood to inform clinical recommendations beyond general awareness.

Growing Pains at the Patellofemoral Joint

Adolescents, particularly physically active males, are vulnerable to a condition called Sinding-Larsen-Johansson disease, which involves irritation and sometimes partial fragmentation of the bottom tip of the kneecap where the patellar tendon attaches. It is essentially a growth-related overuse injury of the patellofemoral unit. It presents as localized knee pain, and ultrasound and MRI are the most effective ways to confirm it. Conservative treatment focusing on activity limitation, rest, and sometimes anti-inflammatory medication generally leads to recovery within two to eight months.³⁵ Parents and coaches sometimes confuse this with Osgood-Schlatter disease, which involves a similar mechanism but at the other end of the patellar tendon (the tibial tuberosity). Both are self-limiting conditions related to growth, but they involve different parts of the extensor mechanism.

The Psychological Side of Patellofemoral Pain

Something that often gets overlooked in discussions of patellofemoral pain is how fear of movement affects recovery. Kinesiophobia, the fear that physical activity will cause re-injury or worsen pain, has a moderate association with worse self-reported function in people with patellofemoral pain. A correlation meta-analysis using individual participant data from nearly 500 people found a meaningful negative relationship between kinesiophobia and function, and the good news is that kinesiophobia appears to be modifiable through interventions.³⁶ If someone with patellofemoral pain is avoiding stairs, squatting, and exercise entirely because they are afraid of damage, addressing that belief directly through education and graded exposure to activity may be as important as the exercise prescription itself.